Lithium secondary battery employing fluorine-substituted cyclohexylbenzene containing electrolytic solution

ABSTRACT

A lithium secondary battery comprising a positive electrode, a negative electrode of artificial graphite or natural graphite and a nonaqueous electrolytic solution having an electrolyte dissolved in a nonaqueous solvent, wherein 0.1 to 20 wt. % of a cyclohexylbenzene having a halogen atom bonded to a benzene ring thereof is contained in the nonaqueous electrolytic solution exhibits large electric capacity and excellent cycle performance.

FIELD OF INVENTION

The present invention relates to a lithium secondary battery havingexcellent battery characteristics in cycle performance, electriccapacity and storage property.

BACKGROUND OF INVENTION

Recently, a lithium secondary battery is generally employed as anelectric source for driving small electronic devices. The lithiumsecondary battery essentially comprises a positive electrode, anonaqueous electrolytic solution, and a negative electrode. A lithiumsecondary battery utilizing a positive electrode of lithium compoundoxide such as LiCoO₂ and a negative electrode of carbonaceous materialor lithium metal is favorably used. As the electrolytic solution for thelithium secondary battery, a carbonate such as ethylene carbonate (EC)or propylene carbonate (PC) is favorably used.

Nevertheless, it is desired to provide a secondary battery showingimproved characteristics in the cycle performance and electric capacity.

A lithium secondary battery utilizing a positive electrode of LiCoO₂,LiMn₂O₄ or LiNiO₂ sometimes shows decrease of electric performancesbecause a portion of the nonaqueous solvent in the nonaqueouselectrolytic solution oxidatively decomposes in the course of chargingand hence the produced decomposition product disturbs the desiredelectrochemical reaction. The decomposition is considered to be causedby electrochemical oxidation of the solvent on the interface between thepositive electrode and the nonaqueous electrolytic solution.

On the other hand, a lithium secondary battery utilizing a negativeelectrode of carbonaceous material of high crystallization such asnatural graphite or artificial graphite also shows decrease of electricperformances because a solvent of the electrolytic solution reductivelydecomposes on the surface of the negative electrode in the course ofcharging. The reductive decomposition also occurs in the repeatedcharging-discharging procedures when EC (which is generally employed asthe nonaqueous solvent of the electrolytic solution) is utilized as thenonaqueous solvent.

Japanese Patent Provisional Publication 10-74537 describes that thecycle performance and electric capacity are improved when a small amountof an aromatic compound such as benzene having a hydrocarbon substituent(e.g., cyclohexylbenzene).

Japanese Patent Provisional Publication 10-112335 describes that thecycle performance is improved when a small amount of a fluorineatom-containing aromatic compound such as fluorobenzene is added to anonaqueous electrolytic solution of a lithium secondary battery.

DISCLOSURE OF INVENTION

The present invention has an object to provide a lithium secondarybattery showing improved battery cycle performance, improved electriccapacity, and improved storability in the charged condition.

The present invention resides in a lithium secondary battery comprisinga positive electrode, a negative electrode of artificial graphite ornatural graphite and a nonaqueous electrolytic solution having anelectrolyte dissolved in a nonaqueous solvent, wherein 0.1 to 20 wt. %of a cyclohexylbenzene having a halogen atom bonded to a benzene ringthereof is contained in the nonaqueous electrolytic solution.

The cyclohexylbenzene having a halogen atom bonded to a benzene ringthereof employed in the invention preferably is a compound having thefollowing formula (I):

wherein X is a halogen atom, and the halogen atom is attached to anoptional position.

Preferred is 1-halogeno-4-cyclohexylbenzene.

DETAILED EXPLANATION OF THE INVENTION

In the cyclohexylbenzene having a halogen atom bonded to a benzene ringthereof (hereinafter referred to as “cyclohexyl-halogenobenzene”)contained in the nonaqueous electrolytic solution containing anelectrolyte dissolved in a nonaqueous solvent, the halogen atompreferably is a fluorine atom or a chlorine atom.

Examples of the cyclohexyl-halogenobenzenes include1-fluoro-2-cyclohexylbenzene, 1-fluoro-3-cyclohexylbenzene,1-fluoro-4-cyclohexylbenzene, 1-chloro-4-cyclohexylbenzene,1-bromo-4-cyclohexylbenzene, and 1-iodo-4-cyclohexylbenzene.

If the content of the cyclohexyl-halogenobenzene in the nonaqueouselectrolytic solution is extremely large, the battery performances maylower. If the content of the cyclohexyl-halogenobenzene is extremelysmall, an expected improvement of the battery performances cannot beattained. Accordingly, the content preferably is in the range of 0.1-20wt. %, more preferably 0.2-10 wt. %, most preferably 0.5-5 wt. %, basedon the amount of the nonaqueous electrolytic solution, so that the cycleperformance can well improved.

Examples of the non-aqueous solvents employed in the electrolyticsolution of the invention are cyclic carbonates such as ethylenecarbonate (EC), propylene carbonate (PC), butylene carbonate (BC), andvinylene carbonate (VC), lactones such as γ-butyrolactone, linearcarbonates such as dimethyl carbonate (DMC), methyl ethyl carbonate(MEC), and diethyl carbonate (DEC), ethers such as tetrahydrofuran,2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane,1,2-diethoxyethane, and 1,2-dibutoxyethane, nitriles such asacetonitrile and adiponitrile, esters such as methyl propionate, methylpivalate, butyl pivalate, octyl pivalate and dimethyl oxalate, amidessuch as dimethylformamide, and compounds containing S═O group such as1,3-propanesultone, glycol sulfite and divinyl sulfone.

The non-aqueous solvents can be employed singly or in combination of twoor more. There are no specific limitations with respect to thecombination of the non-aqueous solvents. Examples of the combinationsinclude a combination of a cyclic carbonate and a linear carbonate, acombination of a cyclic carbonate and a lactone, and a combination ofplural cyclic carbonates and linear carbonates.

Examples of the electrolytes employed in the invention include LiPF₆,LiBF₄, LiClO₄, LiN(SO₂CF₃)₂, LiN(SO₂C₂F₅)₂, LiC(SO₂CF₃)₃, LiPF₄(CF₃)₂,LiPF₃(C₂F₅)₃, LiPF₃(CF₃)₃, LiPF₃(iso-C₃F₇)₃, and LiPF₅(iso-C₃F₇). Theseelectrolytes can be employed singly or in combination of two or more.The electrolyte can be incorporated into the nonaqueous solventgenerally in such an amount as to give an electrolytic solution of 0.1 Mto 3 M, preferably 0.5 M to 1.5 M.

The electrolytic solution of the invention can be prepared, forinstance, by mixing the above-mentioned non-aqueous solvents; dissolvingthe above-mentioned electrolyte in the mixture; and further dissolvingat least one of the above-mentioned cyclohexyl-halogenobenzenes in theresulting mixture.

For instance, the active material of positive electrode is a compoundmetal oxide comprising lithium and cobalt or nickel. The active materialof positive electrode can be used singly or in combination. Examples ofthe compound metal oxides include LiCoO₂, LiNiO₂, and LiCo_(1-x)Ni_(x)O₂(0.10<x<1). These compounds can be employed in an optional combinationsuch as a combination of LiCoO₂ and LiMn₂O₄, a combination of LiCoO₂ andLiNiO₂, and a combination of LiMn₂O₄ and LiNiO₂.

The positive electrode can be manufactured by kneading theabove-mentioned active material of positive electrode, anelectro-conductive material such as acetylene black or carbon black, anda binder such as poly(tetrafluoroethylene) (PTFE) or poly(vinylidenefluoride) (PVDF), to give a positive electrode composition; coating thepositive electrode composition on a collector such as aluminum foil or alath plate of stainless steel; drying and pressing the coatedcomposition, and heating the pressed composition in vacuo at atemperature of approximately 50 to 250° C. for approximately 2 hours.

As the active material of negative electrode, carbonaceous materialcapable of absorbing and releasing lithium (such as artificial graphiteand natural graphite). It is preferred to employ artificial graphite andnatural graphite having a graphite crystal structure in which thelattice distance of lattice surface (002), namely, d₀₀₂, is in the rangeof 0.335 to 0.340 nm (nanometer). The active materials of negativeelectrode can be employed singly or in combination. A powdery materialsuch as the carbonaceous material is preferably used in combination witha binder such as ethylene propylene diene terpolymer (EPDM),poly(tetrafluoroethylene) (PTFE) or poly(vinylidene fluoride) (PVDF).There are no limitations with respect to the preparing method of thenegative electrode. The negative electrode can be prepared by a methodsimilar to that for the preparation of the positive electrode.

There are no specific limitations with respect to the structure of thenonaqueous lithium secondary battery of the invention. For instance, thenonaqueous secondary battery can be a battery of coin type comprising apositive electrode, a negative electrode, and single or pluralseparators, or a cylindrical or prismatic battery comprising a positiveelectrode, a negative electrode, and a separator roll. The separator canbe a known material such as micro-porous polyolefin film, woven cloth,or non-woven cloth.

The lithium secondary battery of the invention exhibits excellent cycleperformance even when it is employed under the charging condition of ahigh terminal voltage of higher than 4.2 V, particularly approximately4.3 V. The discharge terminal voltage can be 2.5 V or higher, moreover2.8 V or higher. There are no specific limitation with respect to thecurrent value, and a constant current of 0.1 to 3 C is generally adoptedfor discharge. The lithium secondary battery of the invention can becharged and discharged within a wide temperature range such as −40 to100° C., but preferably 0 to 80° C.

The lithium secondary battery of the invention may have a safety valveat the sealing plate to obviate increase of the inner pressure.Otherwise, a notch can be provided to the battery case or gasket. Alsoemployable are one or more of known safety elements such as a fuse, abimetal element, and a PTC element, each of which serves as an elementfor obviating overcurrent.

If desired, the lithium secondary battery of the invention can beencased in a battery pack in which plural batteries are arranged inseries and/or in parallel. The battery pack can have a safety elementsuch as a PTC element, a thermostatic fuse, a fuse and/or an electriccurrent breaker, and further a safety circuit (i.e., a circuit capableof monitoring the voltage, temperature and current of the battery ofcombined battery, and then breaking the current).

EXAMPLE 1

[Preparation of Non-Aqueous Electrolytic Solution]

In a nonaqueous solvent of EC:DEC (=3:7, volume ratio) was dissolved 1Mof LiPF₆ to give a nonaqueous electrolytic solution. To the nonaqueouselectrolytic solution was further added 2.0 wt. % of1-fluoro-4-cyclohexylbenzene.

[Manufacture of Lithium Secondary Battery and Measurement of its BatteryPerformances]

LiCoO₂ (positive electrode active material, 80 wt. %), acetylene black(electro-conductive material, 10 wt. %), and poly(vinylidene fluoride)(binder, 10 wt. %) were mixed. To the resulting mixture was furtheradded 1-methyl-2-pyrrolidone. Thus produced mixture was coated onaluminum foil, dried, pressed, and heated to give a positive electrode.

Artificial graphite (negative electrode active material, 90 wt. %) andpoly(vinylidene fluoride) (binder, 10 wt. %) were mixed. To theresulting mixture was further added 1-methyl-2-pyrrolidone. Thusproduced mixture was coated on copper foil, dried, pressed, and heatedto give a negative electrode.

The positive and negative electrodes, a microporous polypropylene filmseparator, and the above-mentioned non-aqueous electrolytic solutionwere employed to give a coin-type battery (diameter: 20 mm, thickness:3.2 mm).

The coin-type battery was charged at room temperature (20° C.) with aconstant electric current (0.8 mA) to reach 4.2 V (terminal voltage) for5 hours. Subsequently, the battery was discharged to give a constantelectric current (0.8 mA) to give a terminal voltage of 2.7 V. Thecharging-discharging cycle procedure was repeated.

The initial charge-discharge capacity was almost the same as thecapacity measured in a battery using an 1M LiPF₆ and EC/DEC (3/7, volumeratio) solvent mixture (containing no additive) [see Comparison Example1].

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 92.9% of the initial discharge capacity (100%).The low temperature performances are also good.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 2

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 5.0 wt. % of 1-fluoro-4-cyclohexylbenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 91.4%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 3

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 0.5 wt. % of 1-fluoro-4-cyclohexylbenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 90.5%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

COMPARISON EXAMPLE 1

In a non-aqueous solvent of EC:DEC (=3:7, volume ratio) was dissolved 1M of LiPF₆ to give a nonaqueous electrolytic solution. To the nonaqueouselectrolytic solution was added no cyclohexylbenzene compound.

Then, a coin-type battery was manufactured by employing the resultingnonaqueous electrolytic solution.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 82.6% of the initial discharge capacity.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 4

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 2.0 wt. % of 1-fluoro-2-cyclohexylbenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 92.4%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 5

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 2.0 wt. % of 1-fluoro-3-cyclohexylbenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 92.0%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 6

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 2.0 wt. % of 1-chloro-4-cyclohexylbenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 89.1%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 7

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 2.0 wt. % of 1-bromo-4-cyclohexylbenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 88.9%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

COMPARISON EXAMPLE 2

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 5.0 wt. % of fluorobenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 82.9%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

COMPARISON EXAMPLE 3

The procedures of Example 1 for preparing a nonaqueous electrolyticsolution and manufacturing a coin-type battery were repeated except forusing 5.0 wt. % of cyclohexylbenzene.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 83.1%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 8

The procedures of Example 1 for manufacturing a coin-type battery wererepeated except for replacing the artificial graphite (i.e., activematerial of the negative electrode) with natural graphite.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 92.6%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 9

The procedures of Example 1 for manufacturing a coin-type battery wererepeated except for replacing the LiCoO₂ (i.e., active material of thepositive electrode) with LiNi_(0.8)Co_(0.2)O₂.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 91.0%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

EXAMPLE 10

The procedures of Example 1 for manufacturing a coin-type battery wererepeated except for replacing the LiCoO₂ (i.e., active material of thepositive electrode) with LiMn₂O₄.

After the 50 cycle charging-discharging procedure, the retention ofdischarge capacity was 92.4%.

The conditions for manufacturing the coin-type battery and the batteryperformances are shown in Table 1.

TABLE 1 Electrode Additive 50% cycle Posi. (amount: Electrolytic Initialcapacity retention Example Nega. wt. %) solution (r.v.) (%) 1 LiCoO₂1-fluoro-4- 1M LiPF₆ 1.02 92.9 Art. cyclohexyl- EC/DEC = 3/7 benzene(2.0) 2 LiCoO₂ 1-fluoro-4- 1M LiPF₆ 1.01 91.4 Art. cyclohexyl- EC/DEC =3/7 benzene (5.0) 3 LiCoO₂ 1-fluoro-4- 1M LiPF₆ 1.01 90.5 Art.cyclohexyl- EC/DEC = 3/7 benzene (0.5) Com. 1 LiCoO₂ None 1M LiPF₆ 1.0082.6 Art. EC/DEC = 3/7 4 LiCoO₂ 1-fluoro-2- 1M LiPF₆ 1.02 92.4 Art.cyclohexyl- EC/DEC = 3/7 benzene (2.0) 5 LiCoO₂ 1-fluoro-3- 1M LiPF₆1.02 92.0 Art. cyclohexyl- EC/DEC = 3/7 benzene (2.0) 6 LiCoO₂1-chloro-4- 1M LiPF₆ 1.01 89.1 Art. cyclohexyl- EC/DEC = 3/7 benzene(2.0) 7 LiCoO₂ 1-bromo-4- 1M LiPF₆ 1.01 88.9 Art. cyclohexyl- EC/DEC =3/7 benzene (2.0) Com. 2 LiCoO₂ fluoro- 1M LiPF₆ 0.99 82.9 Art. benzene(5.0) EC/DEC = 3/7 Com. 3 LiCoO₂ cyclohexyl- 1M LiPF₆ 0.99 83.1 Art.benzene (5.0) EC/DEC = 3/7 8 LiCoO₂ 1-fluoro-4- 1M LiPF₆ 1.02 92.6 Nat.cyclohexyl- EC/DEC = 3/7 benzene (2.0) 9 LiNi_(0.8)Co_(0.2)O₂1-fluoro-4- 1M LiPF₆ 1.14 91.0 Art. cyclohexyl- EC/DEC = 3/7 benzene(2.0) 10  LiMn₂O₄ 1-fluoro-4- 1M LiPF₆ 0.99 92.4 Art. cyclohexyl- EC/DEC= 3/7 benzene (2.0)

INDUSTRIAL UTILITY

The present invention provides a lithium secondary battery havingexcellent battery performances in the cycle performance, electriccapacity, and storage performance.

1. A lithium secondary battery comprising a positive electrode, a negative electrode of artificial graphite or natural graphite and a nonaqueous electrolytic solution having an electrolyte dissolved in a nonaqueous solvent, wherein 0.2 to 10 wt. % of a cyclohexylbenzene having a flourine atom bonded to a benzene ring thereof is contained in the nonaqueous electrolytic solution.
 2. The lithium secondary battery of claim 1, wherein the cyclohexylbenzene having a flourine atom bonded to a benzene ring thereof is a compound having the following formula (I):

wherein X is a flourine atom, and the flourine atom is attached to an optional position.
 3. The lithium secondary battery of claim 2, wherein the cyclohexylbenzene having a flourine atom bonded to a benzene ring thereof is 1-flourine-4-cyclohexylbenzene.
 4. The lithium secondary battery of claim 1, wherein the cyclohexylbenzene having a flourine atom bonded to a benzene ring thereof is contained in the nonaqueous electrolytic solution in an amount of 0.5 to 5 wt. %.
 5. The lithium secondary battery of claim 1, wherein the nonaqueous solvent of the nonaqueous electrolytic solution comprises a combination of a cyclic carbonate and a linear carbonate, a combination of a cyclic carbonate and lactone, or a combination of plural cyclic carbonates and linear carbonates.
 6. The lithium secondary battery of claim 1, which contains vinylene carbonate.
 7. The lithium secondary battery of claim 1, wherein the artificial graphite or natural graphite has a graphite crystal structure having a lattice distance in terms of d₀₀₂ of lattice surface (002) in the range of 0.335 to 0.340 nm. 